1. Technical Field
The invention relates to a sliding element, in particular a piston ring, having a coating on a substrate, which forms at least one running surface.
2. Related Art
Sliding elements such as, for example, piston rings, pistons or cylinder liners in internal combustion engines must work throughout a service life as long as possible, both with as little friction as possible and with a low level of wear. In order to reduce the friction directly coupled with the fuel consumption of the internal combustion engine, it is essentially known to provide such sliding elements with a friction-reducing coating. For this purpose, a coating of DLC (diamond-like carbon) is used, for example, which is essentially known to the person skilled in the art.
Several methods are available for applying a DLC layer. A method often used recently because of the high possible coating rates and relatively low costs is the PACVD method (plasma-assisted chemical vapor deposition), which is described, for instance, in DE 195 13 614 C1.
Another method that is basically used for coating a substrate with a DLC layer is the PVD method (physical vapor deposition), which is described, for instance, in DE 102 03 730 B4.
The disadvantage of DLC layer deposition, in particular by means of the PACVD method, is that the high mechanical residual stresses generated thereby are associated with a relatively poor resistance of the coating, and therefore such layers cannot be used, without difficulties, in sliding elements.
A sliding element is apparent from DE 10 2009 028 504 B3, in which a coating comprises two DLC layers, namely a metal-containing DLC layer and a metal-free DLC layer. The metal-containing DLC layer was thereby applied by means of a PVD method and the metal-free DLC layer was applied by means of a PACVD method, whereby different microstructures can be created in the layers. The PACVD-DLC layer, i.e. the metal-free DLC layer of the prior art, has the high mechanical residual stresses described above. It is assumed in the prior art that these high residual stresses of the PACVD-DLC layer can be compensated by the metal-containing PVD-DLC intermediate layer.
However, the inventors have found that even the method described in DE 10 2009 028 504 B3 is not yet optimal with regard to the resistance of the layer during the entire service life of the engine. Specifically, it may occur occasionally with the sliding element described in the prior art that the layers flake off under heavy loads, and thus the sliding elements deteriorate with respect to their wear properties and friction properties.
A DLC layer system is apparent from DE 100 18 143 B4, which has an adhesive layer on a substrate, a transition layer and a cover layer, in which the adhesive layer can be applied by plasma-assisted coating, in particular by means of a PVD method or a plasma CVD method, and the cover layer of diamond-like carbon can be applied by means of plasma CVD deposition.
JP 2004-10923 A relates to a sliding element having a coating which comprises, from the inside outwards, a chrome layer, a chrome-carbon layer, a DLC layer with graduated hardness and a hard DLC layer.
A piston ring is apparent from DE 2008 016 864 B3, which comprises, from the inside outwards, an adhesive layer, a metal-containing amorphous carbon layer and a metal-free amorphous carbon layer.
DE 10 2009 046 281 B3 relates to a sliding element, in particular a piston ring, in which a CrN layer, an Me (CxNy) layer and a DLC layer are provided from the inside outwards.
Finally, US 2008/0220257 A1 describes a substrate with a coating which comprises, from the inside outwards, an intermediate layer of carbon with a Young's modulus of more than 200 GPa and an amorphous carbon layer with a Young's modulus of less than 200 GPa.